The present invention relates to an additional device for seaplanes and floatplanes according to the precharacterizing clause of Patent claim 1. Fluid-dynamic compromises have to be made with regard to the parts of flying boats, seaplanes, floatplanes and land aircraft provided with floats which are located in the water during take-off and landing; in order to avoid lengthy circumlocutions in each case, these parts will be referred to as floats from now on. Said compromises are based on the following considerations and requirements:
floats are intended to give the aircraft good directional stability on water,
floats should be able to change easily from being pure buoyancy floats to the sliding phase,
floats should have as little drag in air (cD) as possible.
Technical measures which take account in particular of the first twoxe2x80x94hydrodynamicxe2x80x94requirements generally make the cD value worse, while those which improve the cD value generally destroy the buoyancy and sliding capability.
The Coanda effect (Henri Coanda 1886-1972) plays a central role here: on the basis of the continuity law, water flows around curved surfaces on a body and, in the process, its speed is increased resulting, according to Bernoulli""s Law, in a pressure drop under water. Thus, if a float has a dynamically advantageous shape, then this pressure drop results in it normally being drawn into the water, rendering a sliding phase largely impossible, both during take-off and during landing on water.
In contrast, if a float is provided with, for example, a V-shaped lower surface, with a sharp-edged transition into vertical walls, that is to say with a sharp bilge edge, this not only improves the directional stability, but also largely suppresses the Coanda effect. However, this is invariably at the expense of the cD being increased by several times. The compromise to be reached between hydrodynamic and aerodynamic maximization generally therefore needs to be made in favour of hydrodynamics.
This applies in particular to the step or the step-shaped edge on the bottom of the float in virtually all known embodiments: this is hydrodynamically necessary in order to make the water flow turbulent and to allow air to flow in; however, it is aerodynamically disastrous since it likewise produces flow separation, but in this case this is undesirable.
The previously known attempts to overcome this poor compromise are not very numerous:
either the edgexe2x80x94in plan viewxe2x80x94is rounded at the rear
or an arrangement of moving flaps has been proposed which are extended in flight and in the process cover the step-shaped edge, in order to improve the aerodynamic aspect, for example in DE 38 41 878 A1.
The object of the present invention is now to provide a supplement for hydrodynamically optimized floats, which allows the cD of such floats to be improved to a major extent when the aircraft is airborne. A further object is to design such a supplement such that it is light and such that all the components which are subject to wear are easily replaceable. The way in which said object is achieved is described in the characterizing part of claim 1, with regard to the main features of the invention, and in the following claims with regard to further advantageous embodiments.